With the recent trend of information toward multimedia and the like, spatial image type displays for enabling realistic image display have been put under research and development.
Among the spatial image type displays under research are ones of a two-dimensional display system and ones of a three-dimensional display system.
A head mounted display (HMD) is known as a representative example of the spatial image type displays of the two-dimensional display system. The unitized apparatus, a combination of display devices and optical systems, is mounted on the head of a user, i.e., a viewer. When images displayed on the display devices are viewed through the optical systems, a virtual image formed in the front space appears as if an image is floating.
The spatial image type displays of the three-dimensional display system are typically referred to as stereoscopic displays, 3D displays, or the like, being broadly divided into eyeglass types and glassless types.
Among the known representative examples of the 3D displays of eyeglass type are ones of polarized glass type, shutter glass type, and head mounted display type (HMD type).
In the 3D display of polarized glass type, the display surface of the display shows right and left parallax images independently with light having polarized planes orthogonal to each other. A parallax is given to between the eyes of the viewer by using a pair of glasses having right and left orthogonal polarizers, so that a stereoscopic image can be displayed.
In the 3D display of shutter glass type, the display device switches to show right and left images alternately at predetermined cycles. A parallax is given to between the eyes of the viewer by using a pair of glasses which are alternately switched on/off in light transmittance in synchronization with the switching cycles, so that a stereoscopic image can be displayed.
In the 3D display of HMD type, the unitized apparatus, or a combination of display devices, optical systems, and a pair of glasses, is mounted on the head of the viewer. As with the polarized glass type and the shutter glass type, parallax images are given to the eyes of the viewer so that a stereoscopic image can be displayed.
Among the known representative examples of the 3D displays of glassless type are ones of parallax barrier type, lenticular lens type, and electronic holography type.
In the 3D display of parallax barrier type, the display surface of the display shows right and left images in strips alternately. The right and left images are viewed by the right and left eyes through a slit plate which is arranged in front of the display surface. Parallax images are thus given to the eyes of the viewer so that a stereoscopic image can be displayed.
In the 3D display of lenticular lens type, a screen plate, or an integration of small semi-cylindrical lenticular lenses, is arranged in front of the display surface of the display device instead of the slit plate which is arranged in the parallax barrier type. Right and left images are shown to the right and left eyes through the lenticular lenses, so that parallax images can be given to the eyes of the viewer to show a stereoscopic image.
The 3D display of electronic holography type uses a hologram, the three-dimensional shape of a subject to be recorded as interference fringes. This hologram is irradiated with illumination light so that it is reproduced as a real image or virtual image by the resulting diffraction light, thereby showing a stereoscopic image.
For the case of the head mounted display, i.e., the spatial image type display of the two-dimensional display system described above, the viewer must wear the heavy, large apparatus on the head and cover the eyes with the optical systems of the apparatus. Thus, there have been pointed out such drawbacks that wearing the apparatus is troublesome and inconvenient, and can cause eyestrain.
The 3D displays of eyeglass type are also known to have such drawbacks that the glasses must be worn troublesomely and inconveniently, and that eyestrain can occur since the right and left eyes view the parallax images alternately.
Among the 3D displays of glassless type, those of parallax barrier type and lenticular lens type are known to have a problem of extremely difficult application to general uses since the viewing angle at which the viewer can see precise binocular parallax images is extremely narrow and the stereoscopic image may disappear even with a slight head movement.
For the 3D display of electronic holography type, it is found difficult to develop a simple, small-sized holography device for realizing holography. A technical breakthrough has been expected, leaving an extreme difficulty in application to general uses.
In this research-and-development stage of spatial image type displays, an epoch-making research has been published. The following provides the display principle underlying the research.
FIG. 17 shows the display principle schematically. A front display surface PL1 and a rear display surface PL2 for displaying images toward the viewer are arranged opposite to each other. Two images displayed on the respective display surfaces PL1 and PL2 are shown to the viewer to produce such an illusion that a stereoscopic image appears in the space between the front display surface PL1 and the rear display surface PL2.
That is, the front display surface PL1 and the rear display surface PL2 both make display surfaces for emitting light toward the viewer by themselves. Besides, the image of the rear display surface PL2 is transmitted through the front display surface PL1 so that the viewer can see both the images of the front display surface PL1 and the rear display surface PL2.
The front display surface PL1 and the rear display surface PL2 show the same image along the direction of the optical axis (the direction of emission of light) though in different brightnesses. When the viewer views the overlaid images, the foregoing illusion shows a stereoscopic image as if lying in the space between the front display surface PL1 and the rear display surface PL2, thereby achieving stereoscopic display.
Here, when the front display surface PL1 shows an image F11 of certain brightness and the rear display surface PL2 shows an image F21 of the same shape, having brightness higher than that of the image F11, an image appears closer to the rear display surface PL2. When the front display surface PL1 and the rear display surface PL2 show images F12 and F22 of the same brightness and the same shape, an image appears at an intermediate position between the front display surface PL1 and the rear display surface PL2. When the front display surface PL1 shows an image F13 of higher brightness and the rear display surface PL2 shows an image F23 of lower brightness, an image appears closer to the front display surface PL1.
Thus, the display images on the front display surface PL1 and the rear display surface PL2 can be adjusted in brightness to achieve stereoscopic display.
To realize a spatial image type display based on the display principle of FIG. 17 requires, however, such components as a complicated optical system for displaying images on the front display surface PL1 and the rear display surface PL2. It has therefore been difficult to reduce the apparatus in weight, thickness, size, etc., with a problem of extremely difficult application to general uses.
For more details of the problem, FIG. 18 schematically shows possible configuration of a spatial image type display to which the display principle of FIG. 17 is applied.
This spatial image type display includes a liquid crystal display LCD1, a liquid crystal display LCD2, and a half mirror BS. The liquid crystal display LCD1 corresponds to the front display surface PL1. The liquid crystal display LCD2 corresponds to the rear display surface PL2. The half mirror BS reflects and transmits the images displayed on the display surfaces of the liquid crystal displays LCD1 and LCD2, respectively, and emits the resultant toward the viewer.
For the sake of self luminescence of the respective images, the liquid crystal displays LCD1 and LCD2 are provided with backlights BL1 and BL2, respectively.
According to this configuration, the images F11, F12, F13, etc. formed by the liquid crystal display LCD1 are reflected at the half mirror BS and emitted toward the viewer, realizing the front display surface PL1 shown in FIG. 17. The images F21, F22, F23, etc. formed by the liquid crystal display LCD2 are transmitted through the half mirror BS and emitted toward the viewer, realizing the rear display surface PL2.
Nevertheless, since the display surfaces of the liquid crystal displays LCD1 and LCD2 must be directed toward the half mirror BS, the liquid crystal display LCD1 is necessarily laid down along the depth direction. This increases the thickness d in the depth direction as shown in the diagram, causing a problem of yet greater size of the entire apparatus.
Note that FIG. 18 shows the basic configuration of the apparatus alone for convenience of explanation. In fact, the complicated optical system including the half mirror BS inevitably increases the overall weight of the apparatus. Consequently, there have been such problems that an increase occurs not only in the aforementioned thickness d in the depth direction but in width and height as well, and that the optical system is susceptible to vibrations etc. and is difficult to adjust for improved optical precision.